Is there any way (possibly through external add-ins) to organize tasks in hierarchy (with tasks and subtasks) in Microsoft Outlook? What I need is something similar to this SharePoint Web Part.
Yes, I use Taskline, which does exactly that. It has not yet been updated for Outlook 2010's ribbon interface, but it still works okay. It's takes a little effort to figure out.
I cannot think why Microsoft has not added this functionality to Outlook yet, other than to push Microsoft Project.